Weather /

Anyone who successfully completes a flight with an FAA-designated examiner to obtain a private pilot certificate knows a great deal more about aeronautics, navigation, FAA rules, and aviation weather than the commercial air traveler. While pilots have to know more about weather than most people, they don’t have to have the deep knowledge expected of someone with a degree in meteorology.

Mercury barometers are used to measure atmospheric pressure, also known as barometric pressure. Meteorologists use barometers to predict short-term changes in the weather. From the AMS Weather Book by Jack Williams, used with permission of the American Meteorological Society (right).

Although you don’t need to be thinking constantly about atmospheric pressure before and during a flight, it’s often the background to the things that you should think about. Knowing a little more about it could help you make better preflight and in-air decisions.

A pilot who knows how an aircraft’s altimeter converts air pressure readings into height indications will know how and why altimeter readings could be misleading. A pilot who understands what is going on in areas marked with an “H” (for high atmospheric pressure) or “L” (for low atmospheric pressure) on weather maps will have a better idea of what kind of weather to expect in or near these areas. For more on air pressure differences, see “Weather: Watching the Winds” (March 2014 Flight Training).

ATMOSPHERIC PRESSURE DEFINITIONS. Understanding these definitions is a good way to begin understanding atmospheric pressure.

Station pressure is the reading directly from a barometer, which is not mentioned in regular weather reports. Weather stations report two pressure figures: sea-level pressure and the altimeter setting.

Sea-level pressure, which is a calculated figure, is based on station pressure. It amounts to what the pressure would be at sea level at a particular time and weather station if it could be measured directly, such as at the bottom of a hole dug down from the surface to sea level. The calculation uses current pressure and temperature as well as the station’s elevation.

Meteorologists use sea-level pressure to draw areas of high and low pressure on weather maps. If station pressures were used to draw maps, the lowest pressures would always be over the highest elevations, falsely showing the low-pressure areas at the centers of storms over all high elevations.

The altimeter setting is a calculated pressure, like sea level pressure, but without using the air’s temperature. When an altimeter is properly calibrated and the pilot knows how to use it, including how to update its setting before takeoff and while flying away from the departure airport, it will help the pilot avoid mountains, towers, and other fixed hazards.

PRESSURE IS MEASURED IN INCHES. In the United States we measure pressure, such as that of a fluid in a hydraulic system, in units such as pounds per square inch. The metric system uses newtons per square meter.

Yet, as a pilot you’re used to hearing about “inches of mercury”—both in weather observations and when adjusting your altimeter to the setting reported by an automated weather station or air traffic controller.

This is a historical hangover. The first measurements of atmospheric pressure were made using mercury barometers, which indicate the atmospheric pressure by how high it pushes mercury up into a tube that’s closed at the top. The United States uses inches of mercury for altimeter settings and National Weather Service observations intended for the general public.

In the early twentieth century, scientists were transforming the study of weather from a mainly observational science to a mathematical one that uses formulas to describe how various forces affect the weather. They needed to describe atmospheric pressure as a pressure, not as a length.

Weather scientists and forecasters—almost all work in the metric system these days—began using the millibar, with 1,000 millibars equal to a force of 100,000 newtons per square meter.

In most of the world, including in Canada, the term millibars is being replaced by hectopascals (hPa) for public reports and forecasts, with 1,000 hectopascals being the same as 1,000 millibars. But the U.S. National Weather Service uses millibars for upper air pressure reports and forecasts.

AIR PRESSURE IS IMPORTANT. The fact that both air pressure and density decrease with altitude has important consequences for aircraft and the people in them. Whether piston or jet, an aircraft’s engine creates power by burning fuel, and that combustion requires oxygen from the air. As you go higher, oxygen continues to make up about 21 percent of the air, but since there are fewer molecules of all kinds in each cubic foot of air, engines have less oxygen available than at lower altitudes. Eventually you will reach an altitude at which the engine is using all available oxygen; none remains to produce more power to take the aircraft higher.

As you gain altitude, you also lose lift in the thinner air. In very basic terms, a certain number of air molecules must flow around your airplane’s wings each second to create the lift that keeps the airplane in the air. As the air becomes less dense with altitude, the decrease in drag helps make up for the thinner air—but only to a point.

Humans, too, run into trouble when the air grows too thin. Just like an airplane’s engine, the cells of our bodies need oxygen to burn our fuel—food—to keep our bodies functioning. This is why most airplanes that normally fly much higher than 10,000 feet are pressurized.

Air is pumped into the cabin, making the pressure and density the same as a lower altitude at which our bodies receive enough pressure to work well. For that reason, high-flying airplanes that aren’t pressurized need to be equipped with oxygen masks.

WEATHER COMPLICATES AIR PRESSURE. If the Earth didn’t have weather, air pressure would be easier to understand. Pressure would decrease at a regular rate as you ascended, and that would be it. But, as we know, the weather brings changes in air pressure and the rate at which it decreases with altitude.

Compared to the changes in air pressure seen when an airplane climbs or descends, the pressure differences that create winds and storms are small. But these relatively small changes have huge consequences.

For example, on August 13, 2004, when Hurricane Charley was off  Florida’s west coast with winds up to 140 mph, the pressure in its eye was 28.61 inches of mercury. The pressure on the coast 60 miles away was 29.80 inches of mercury—quite a wind created by a difference of only 1.19 inches of mercury.

Air pressure differences cause the winds to blow as air moves from high to low pressure. The bigger the difference, the faster the winds blow. This is why the lower the pressure at its center, the stronger a storm will be. It’s also why measurements of the air pressure and how it is changing are an important part of all weather observations.

Forecasters need to know what the current air pressures are doing in order to forecast what the pressures will be in the future. When forecasters have a good handle on future air-pressure patterns, they are able to accurately predict wind speeds and directions, and thus what kind of weather to expect when you
go flying.